In the arm tefillin, the four passages are written on a
single scroll. In the head tefillin, they're on four separate scrolls.

3. The Boxes

The scrolls are inserted into boxes made of leather that has been
painstakingly pressed into the tefillin's perfectly smooth planes and precise geometrical shape, and painted black.

The upper part of the tefillin box is a perfect cube; the lower part is a flatter box that is wider than the upper part.
On one side, the lower part extends further than the other sides and has a slit through which the tefillin strap is threaded.

The hand tefillin has one large compartment, in which a single scroll
inscribed with the four portions is inserted. The head tefillin has four
compartments, for its four scrolls, and has a raised Hebrew letter shin on each side.

4. The Straps

The straps are made of leather painted black on one side. They are threaded through the lower part of the boxes and knotted.

The head tefillin has a large, fixed loop to fit the head. The hand tefillin has a smaller, adjustable loop to tie on the upper arm.

The head strap's knot is in the shape of the Hebrew letter daled; the hand tefillin is knotted in the shape of the Hebrew letter yud. (Together, shin, daled,
yud spell Sha-da-i -- one of the names of G-d.)